1. Field of the Invention
This invention relates to a hydraulic control system for controlling the operation of a variable camshaft timing (VCT) system. More specifically, the present invention relates to a control system which utilizes a position sensor mounted to the spool valve position and a control loop controlling the position of the spool valve.
2. Description of Related Art
U.S. Pat. No. 5,167,206 discloses a helical spline type phaser, which uses a hydraulic piston to move splines axially, which causes the sprocket and cam to move radially. A motion-sensing rod is surrounded by a coil, which forms an electromagnetic pick-up on rod position.
The control system disclosed in both U.S. Pat. Nos. 5,172,659 and 5,184,578 utilizes hydraulic force on both ends of a spool valve. U.S. Pat. No. 5,184,578 shows the control system, in which crank and cam positions are sensed and a Pulse-width Modulated Solenoid moves a spool valve to control the actuation of the phaser, with a closed-loop control measuring the phase difference between cam and crank, and operating the spool valve accordingly.
U.S. Pat. No. 5,497,738 uses a variable force solenoid to control the phase angle using a center mounted spool valve. This type of variable force solenoid can infinitely control the position of the phaser. The force on the end of the vented spool valve located in the center of the phaser is applied by an electromechanical actuator, preferably of the variable force solenoid type, which acts directly upon the vented spool in response to an electronic signal issued from an engine control unit (xe2x80x9cECUxe2x80x9d) which monitors various engine parameters.
The ECU receives signals from sensors corresponding to camshaft and crankshaft positions and utilizes this information to calculate a relative phase angle. A closed-loop feedback system which corrects for any phase angle error is preferably employed. The use of a variable force solenoid solves the problem of sluggish dynamic response. Such a device can be designed to be as fast as the mechanical response of the spool valve, and certainly much faster than the conventional (fully hydraulic) differential pressure control system. The faster response allows the use of increased closed-loop gain, making the system less sensitive to component tolerances and operating environment.
FIG. 1 shows a block diagram of a further development of the control system shown in U.S. Pat. No. 5,497,738. The Engine Control Unit (ECU) (1) decides on a phase set point (2), based on various demands on the engine and system parameters (temperature, throttle position, oil pressure, engine speed, etc.). The set point is filtered (3) and combined (4) with a VCT phase measurement (12) in a control loop with a PI controller (5), phase compensator (6), and anti-windup logic (7). The output of this loop is combined (9) with a null duty cycle signal (8) into a current driver (10), whose output is combined (13) with a dither signal (11) to provide current (320) to drive the variable force solenoid (VFS)(201). The VFS (201) pushes upon the spool valve (192) which is located in the center of the phaser (14). The spool valve (192), in turn, controls fluid (engine oil) to activate the VCT phaser (14), either by applying oil pressure to the vane chambers or by switching passages to allow cam torque pulses (15) to move the phaser (14), as shown in the patents cited above. The cam position is sensed by a cam sensor (20), and the crank position (or the position of the phaser drive sprocket, which is connected to the crankshaft) is also sensed by sensor (21), and the difference between the two is used by a VCT phase measurement circuit (19) to derive a VCT phase signal (12), which is fed back to complete the loop.
One problem with this system is that the variable force solenoid (201) and the spool valve (192) have both frictional and magnetic hysteresis. This can cause the null position of the spool valve (192) to vary, as the position (310) of the spool valve with increasing current (320) can be different than the position (310) of the spool valve (192) with decreasing current (320). This variable position is shown in graphs (330) and (335) in FIG. 1.
Therefore, there is a need in the art for a system and method which minimizes errors due to hysteresis.
The cam phaser of the present invention includes a variable force solenoid with spool position feedback to control the position of a center mounted spool valve and control the phase angle of the cam mounted phaser. A position sensor is mounted to the spool valve position such that a control loop controls the position of the spool valve. A second, outer loop controls the phaser angle. An offset is preferably added to the spool valve position to move the spool valve to its steady state or null position. This null position is required so that the spool can move in to move the phaser in one direction and move out to move the phaser in the other direction. This type of system reduces any frictional or magnetic hysteresis in the spool and solenoid control system.